Mini-Review Modulation of inflammation by interleukin-27 Markus Bosmann* and Peter A. Ward†,1 *Center of Thrombosis and Hemostasis and Department of Hematology and Oncology, University Medical Center, Mainz, Germany; and †Department of Pathology, University of Michigan Medical School, Ann Arbor, Michigan, USA RECEIVED FEBRUARY 27, 2013; REVISED JUNE 14, 2013; ACCEPTED JULY 9, 2013. DOI: 10.1189/jlb.0213107 ABSTRACT lyze structure, signaling, and biological effects of IL-27. De- A growing body of evidence suggests an essential role spite accumulating literature on IL-27, several aspects of this of the heterodimeric cytokine, IL-27, for regulating im- cytokine remain enigmatic. In this review, we will provide a munity. IL-27 is composed of two subunits (p28 and perspective of the current knowledge of IL-27 functions and EBI3) and is classified as a member of the IL-12 family implications during autoimmune and infectious diseases. We of cytokines. APCs have been recognized as a major present C5a as a selective regulator of IL-27 gene expression. cellular source of IL-27 following activation with micro- bial products or IFNs (types I and II). In this review, we Furthermore, we dissect the available literature, classifying describe the current knowledge of the implications of IL-27 as a proinflammatory or anti-inflammatory factor during IL-27 during the pathogenesis of infectious and autoim- acute and chronic models of inflammation. mune diseases. Experimental studies have used genet- ically targeted IL-27RAϪ/Ϫ mice, EBI3Ϫ/Ϫ mice, and p28Ϫ/Ϫ mice or involved study designs with administra- tion of bioengineered IL-27/IL-27RA homologs. STRUCTURE OF IL-27 Whereas many reports have described that IL-27 sup- presses inflammation, we also review the current litera- The heterodimeric, glycosylated protein IL-27 is assembled by ture, suggesting promotion of inflammation by IL-27 in the subunits p28 (also known as IL-27p28, IL-27A, IL-30) and some settings. Recent advances have also been made EBI3 [1]. EBI3 is a 229-aa protein (mouse: 228 aa) encoded in understanding the cross-talk of cleavage products of on human chromosome 19 (mouse: chromosome 17). The the complement system with IL-27-mediated immune subunit p28 consists of 243 aa (mouse: 234 aa) and is located responses. Additional data on IL-27 have been ob- on human chromosome 16 (mouse: chromosome 7). Whereas tained recently by observational studies in human pa- the predicted structure of p28 is a four-helical cytokine, EBI3 tients with acute and chronic inflammatory diseases. Collectively, the findings from the past decade identify is considered to be a soluble receptor [2]. The predicted in- IL-27 as a critical immunoregulatory cytokine, espe- terface of p28/EBI3 is composed of noncovalent bonds of dis- cially for T cells, whereas some controversy is fueled by tinct hydrophobic and polar properties involving specific results challenging the view of IL-27 as a classical si- amino acid residues of p28 (Trp97) and EBI3 (Phe97, Asp210, lencer of inflammation. J. Leukoc. Biol. 94: 1159–1165; Glu159) [2]. IL-27 displays sequence homology with members 2013. of the IL-6/IL-12 family of cytokines. Genes encoding for ho- mologs of IL-27 subunits have been identified in nearly 20 mammalian species, whereas little information is available for other vertebrates. IL-27 was first described by Pflanz and colleagues [1] in 2002 In humans and rodents, other binding partners for p28 or as a cytokine with activity on T cells. Notably, the discovery of EBI3 exist (Fig. 1). EBI3 can associate with the subunit p35 IL-27 may have been complicated by the fact that the different (IL-12A), resulting in the cytokine IL-35 [3]. In addition, p28 protein chains of IL-27 are encoded on independent gene loci. Over the past decade, scientific efforts have aimed to ana- appears to have another binding partner, CLF-1 [4]. The exis- tence and biological relevance of homodimers (EBI3/EBI3; p28/p28) and isolated secreted subunits of p28 or EBI3 are not entirely clear, but such molecules may act as natural antag- Abbreviations: Ϫ/Ϫϭdeficient, AhRϭaryl hydrocarbon receptor, C3a/C4a/ onists of heterodimeric IL-27 [5]. C5aϭcomplement component C3a/C4a/C5a, CLF-1ϭcytokine-like factor 1, EAEϭexperimental autoimmune encephalomyelitis, EBI3ϭEBV-induced gene 3, Egr-2ϭearly growth response gene 2, Foxp3ϭforkhead box p3, H1N1ϭinfluenza A, HSVϭHerpes simplex virus, IRFϭIFN regulatory factor, p28ϭIL-27 subunit p28, IL-30, SNPϭsingle nucleotide polymorphism, ϭ ϭ TNBS 2,4,6-trinitrobenzene sulfonic acid, Tr1 type 1 regulatory T cell, 1. Correspondence: Dept. of Pathology, University of Michigan Medical Tregϭregulatory T cell, TRIFϭToll/IL-1R domain-containing adapter-induc- School, 1301 Catherine Rd., Ann Arbor, MI 48109-5602, USA. E-mail: ing IFN-␤ [email protected] 0741-5400/13/0094-1159 © Society for Leukocyte Biology Volume 94, December 2013 Journal of Leukocyte Biology 1159 and II IFNs [23–25]. IFNs may also be a relevant trigger to initi- ate IL-27 production during autoimmune diseases. Another pathway to activate IL-27 production in myeloid cells is activation of the costimulatory molecule, 4-1BB (CD137), a member of the TNFR family, using agonistic anti- bodies [26]. Figure 1. Subunits of IL-12 family members and their known associa- Extracellular ATP, which is viewed as a “danger signal” fol- tions as heterodimers. p35, IL-12A; p40, IL-12B. lowing tissue injury, mediates suppression of p28 and less po- tently of EBI3 [27]. These effects involve signaling via puriner- gic (P2) receptors (e.g., P2Y11 receptor) [27]. In addition, IL-27R AND SIGNALING IL-27 appears to be silenced following activation of the com- plement system. IL-27 binds to a heterodimeric receptor complex, which is composed of a ligand-binding chain, IL-27RA (WSX-1, TCCR), and an additional signal-transducing chain, gp130 [6, 7]. Of IL-27 AND THE COMPLEMENT SYSTEM note, IL-27RA includes a cytoplasmatic domain with Box 1 mo- The complement system is part of innate immune defenses tif binding sites for JAK1/2, which contributes to signal trans- and is required for immediate clearance of pathogens [28, duction of the IL-27RA/gp130 dimer [6]. The common gp130 29]. In addition, complement activation products, such as C3a, chain is shared by several other receptors for signaling of cyto- C4a, and C5a, act as potent anaphylatoxins to orchestrate the kines, such as IL-6, IL-11, and leukemia inhibitory factor [8]. activation and chemotaxis of various immune cells [29]. Com- The situation becomes even more complex with heterodimers plement cleavage products induce complex signaling cascades of CLF-1/p28 binding to a trimeric receptor complex com- to maintain immune surveillance and tissue homeostasis [28]. posed of IL-27RA/gp130/IL-6RA [9]. For instance, the classical receptor for C5a (C5aR) is ex- Whereas gp130 is found widely in various tissues, the expres- pressed abundantly on PMN leukocytes and to a lesser extent, sion of IL-27RA appears to be more restricted to immune on macrophages [15]. Ligation of C5a with C5aR suppresses cells, such as T, B, NK, plasma, and possibly cells of the my- gene expression and release of p28 from F4/80ϩ macrophages eloid lineage [7, 10, 11]. However, IL-27RA may also be ex- [15, 30, 31]. Likewise, the C5a degradation product, C5adesArg, pressed on transformed, malignant, nonimmune cells [12]. has retained some functionality to antagonize p28 production IL-27, via IL-27RA/gp130, activates JAK and STAT1/STAT3. from macrophages, presumably with less potency than C5a STAT1 and STAT3 modulate the activity of downstream T cell [15]. During endotoxic shock, higher amounts of p28 are gen- lineage-specific transcription factors, such as T-bet (Th1), erated in C5aRϪ/Ϫ mice compared with WT mice, whereas GATA-3 (Th2), and retinoic acid receptor-related orphan re- no such effects have been observed in mice deficient in the ␥ ceptor t (Th17). In addition, IL-27 signaling activates p38 second C5a receptor, C5L2 [15]. In cell culture experiments, MAPK and recruits the transcription factors c-Maf and the lower frequencies of F4/80ϩp28ϩ macrophages were ob- AhR [13, 14]. served, when LPS (TLR4 activation) was combined with C5a treatment. The inhibitory effects of C5a on p28 gene expres- REGULATION OF IL-27 GENE EXPRESSION BY MICROBIAL PRODUCTS AND ENDOGENOUS FACTORS IL-27 is expressed mainly by APCs (macrophages and DCs) after encountering microbial products. Abundant quantities of IL-27 are secreted after activation of PRRs, such as TLR3 (polyinosinic:polycytidylic acid), TLR4 (LPS), and in some re- ports, TLR7 (loxoribine) and TLR9 (CpG) [15–17]. Irradiated gram-negative bacteria (e.g., Escherichia coli) induced expres- sion of p28 in human monocyte-derived DCs [18]. Similar re- sults were obtained for IL-27 induction by viable Salmonella enteritidis [19]. Production of IL-27 has been noted during vi- ral, bacterial, protozoan, and fungal infections in vivo [17, 20– 22] (Fig. 2). TLRs recruit the adaptor molecules MyD88 and TRIF for initia- tion of downstream signaling cascades. After activation of TLR4 with LPS, MyD88 and TRIF are required for full activation of Figure 2. Regulation of IL-27 p28 gene expression by the complement system in the context of infection. Viral infections activate the TLR3 IL-27 gene expression [23, 24]. Subsequently, transcription fac- ␬ pathway, whereas bacterial infections predominantly activate the TLR4 tors, such as NF- B (c-Rel), IRF-1, IRF-3, and IRF-9, bind to the pathway. Simultaneous, complement activation results in generation of promoter region of p28 [23–25]. An important amplifying circuit the anaphylatoxin, C5a, which recruits PI3K-Akt for selective inhibition for IL-27 production has been identified as endogenous types I of the TLR4 pathway to limit IL-27 production.